Cartilage is a highly hydrated connective tissue with chondrocytes embedded in a dense extracellular matrix made of, for example, collagen, proteoglycan and water. Although the biochemical composition of cartilage differs according to types, there are mainly three types of cartilage present in a mammal, which include: articular or hyaline cartilage, fibrocartilage, and elastic cartilage. Hyaline cartilage is predominantly found on the articulating surfaces of articulating joints and contains type II collagen and proteoglycans. It is found also in epiphyseal plates, costal cartilage, tracheal cartilage, bronchial cartilage, and nasal cartilage. Fibrocartilage is mainly found in menisci, the annulus fibrosis of the intervertebral disc, tendinous and ligamentous insertions, the symphysis pubis, and insertions of joint capsules. The composition of fibrocartilage is similar to hyaline cartilage except that fibrocartilage contains fibrils of type I collagen that add tensile strength to the cartilage. Elastic cartilage is present in the pinna of the ears, the epiglottis, and the larynx and is similar to hyaline cartilage except that it contains fibers of elastin.
One of the most common cartilage injuries is damage to the fibrocartilage in the knee joint. Meniscal tears are common in young individuals due to sports-related injuries, as well as in older population suffering from degenerative joint diseases. Meniscal allograft transplantation is one of the available treatment options for patients with meniscal tear. Despite some positive results, issues with tissue rejection, disease transmission and a lack of long-term data have limited the use of this approach.
Diseased or traumatized intervertebral disc is another common fibrocartilage injury. The damage on the annulus can cause pain and possible disc herniation that can compress nerves or the spinal cord resulting in arm or leg pain and dysfunction. Recent advances in molecular biology, cell biology and material sciences have opened a new emerging field for cartilage repair.
However, the most common cartilage injury is articular cartilage injury often as a result of sports related trauma. Due to its avascular nature, articular cartilage has very limited capacity for repair. Approximately 500,000 arthroplastic or joint repair procedures are performed each year in the United States. These procedures include approximately 125,000 total hip and 150,000 total knee arthroplastic procedures (Chen, et al., Repair of articular cartilage defects: Part 1, Basic Science of Articular Cartilage Healing, Amer. J. Orthopedics 1999:31-33). Articular cartilage is a complex tissue involving biomechanical function and associated physical stimuli inside the articular cartilage. Articular cartilage is an inhomogeneous material (tissue) and surface loading is converted to mechanical and electrochemical signals by the extracellular matrix through hydraulic and osmotic pressures, fluid and solute/ion flows, matrix deformations and electrical fields (Mow, Van C. and C. C-B. Wang, Some bioengineering considerations for tissue engineering of articular cartilage. Clinical and Orthopedics and Related Research. 1999, Number 367s, S204-S223).
Unfortunately, chondral defects may not heal, especially when the defect does not penetrate the subchondral bone. A wide variety of surgical procedures are in current use or have been proposed for use to repair chondral defects attempt to prompt the resident cellular population to become more metabolically active thereby promoting new matrix synthesis, however, for the most part, these surgical procedures do little more than provide temporary relief of pain.